[Physiology of Langerhans cells and their potential role in oral pathology]

Langerhans cells represent a minor epidermal cell population in mammals. They are also observed in squamous epithelia of the oesophagus, vagina and cervix, as well as in oral epithelia. They occur in higher density in the non keratinized epithelium. These cells are characterized by a dendritic pattern, a clear cytoplasm and ultrastructurally by the presence of Birbeck granules. They are usually located in a suprabasal position. Their bone marrow origin is now well established. Surface markers and functional properties identify them as belonging to the macrophage/monocyte lineage. Langerhans cells can be identified in tissue sections by immunofluorescence or immunoperoxidase techniques using monoclonal antibodies directed against surface antigens such as class II histocompatibility antigens, T6 marker, or possibly T4 marker. There is also a cytoplasmic marker, the S-100 protein. A renewed interest in Langerhans cells comes from evidence of their role in the cutaneous immune response. At present these cells are considered as dendritic cells expressing a high density of class II histocompatibility antigens and behave as very potent antigen presenting cells that activate mainly helper T lymphocytes. However, experimental data on antigen processing and interleukin 1 secretion are still lacking. This review also examines the oral pathology literature with respect to modifications in the number or localization of Langerhans cells and their proximity to T lymphocytes, for example in lichen planus, Behcet's syndrome, erythema multiforme, gingivitis and oral carcinoma. Histiocytosis X represents a particular case in which the Langerhans cell itself is affected.